This invention relates to measurement of corrosive characteristics of a fluid and more specifically concerns improved compensation for errors in such measurement that are due to one or more of several different environmental variables.
A common method of measurement of corrosion characteristics of a corrosive environment, such as a liquid or gas confined in a pipe or other vessel, employs resistance measurement of a metallic corrodible test element to indicate, by change in electrical resistance, the amount of metal that has been lost by corrosion over a period of time. The term corrosion, as employed in this description, also covers erosion, a process that is also measured by electrical resistance methods such as illustrated in U.S. Pat. No. 3,104,355 to Holmes et al. A widely used device for measurements of this type is known as a Corrosometer probe manufactured by Rohrback Corporation, assignee of this application. In a probe of this type, a sacrificial electrical resistance element is inserted into a corrosive environment. The probe also carries a reference element made of the same material as the test element. Alternating current is passed through the elements and electrical resistance of each is measured while and after the probe has been immersed in an environment of which corrosive tendencies are to be monitored. Because the resistance of the test element changes with the amount of metal of the element, measurement of resistance changes provides an indication of corrosion. However, because resistance of the metal also changes with temperature, a reference element is provided, made of the same material as the test element and having the same temperature resistance characteristics. Changes in resistance of the test element that are due to long-term, relatively slow temperature variation, are eliminated by comparison of resistances of the test and reference element. Commonly, the ratio of resistance of test element to resistance of reference element is measured by means of a Wheatstone bridge or equivalent.
In U.S. Pat. No. 4,338,563 for Corrosion Measurement with Secondary Temperature Compensation, there is described a method of compensating for dynamic or short-term variation of temperature of the environment by directly measuring temperature of both test and reference elements and compensating the corrosion signal according to the directly measured short-term temperature difference of the elements.
Corrosion measurements made with electrical resistance instruments, even with the secondary compensation of U.S. Pat. No. 4,338,563, are widely useful for measurement of relatively long-term (weeks or months) corrosion rates, but provide little or no useful accurate information with respect to relatively slight amounts of metal depletion that occurs in shorter periods of time such as hours or days. Metal losses for many systems are in the order of millionths of an inch per hour, with concomitant test element resistance changes in tenths of a micro-ohm. Measurement of such low level changes for accurately determining small amounts of corrosion are exceedingly difficult, if not impossible, because of the amount of unwanted noise in the desired corrosion signal. Such noise includes both correlated noise (variations of the corrosion output signal that are some function of a variable of the environment being monitored) and uncorrelated noise (variations of output signal not related to environment variables). The environmental variables that cause correlated noise actually cause changes in the resistance of the test or reference elements to thereby falsely signal a different rate of corrosion.
U.S. Pat. No. 4,338,563, to Rhoades et al., and U.S. Pat. No. 4,217,544, to Schmidt, provide compensation for temperature induced errors by measuring both or one of test and reference element temperatures at selected points. However, since there is a continuous temperature gradient along the probe between the inner end of the probe, at or within the corrosive environment, and the outer end of the probe, outside of the environment, temperature measurement at a selected point on one or both elements does not adequately compensate for output signal error due to temperature induced noise.
There exist other sources of correlated noise, that is, signal errors caused by other environmental variables, which errors have not heretofore been eliminated. These additional sources of correlated noise include strain on the test element resulting from bending experienced by relatively long, narrow test elements, and induced by fluid flow velocities, and strain on the thin plate or membrane type test element of a flush probe that is subject to variation of fluid pressure.
Accordingly, it is an object of the present invention to provide corrosion measurement of increased precision.